1. Field of the Invention
The present invention relates to a multilayered film coated on a wear-resistant member such as a cutting tool, a sliding member, or a mold. Particularly, the present invention relates to a multilayered film having excellent adhesion to a substrate of the cutting tool or sliding member, and excellent wear resistance and heat resistance, and a method for producing the multilayered film. The multilayered film of the present invention can be used for the above-described various applications. However, application to a cutting tool will be mainly described below as a representative example.
2. Description of the Related Art
In general, cutting tools and sliding members required to have excellent wear resistance and low friction comprise a substrate comprising high-speed steel or a cemented carbide, and a hard film comprising titanium nitride or titanium aluminum nitride and formed on the surface of the substrate by a physical vapor deposition method (referred to as a “PVD method” hereinafter) or a chemical vapor deposition (referred to as a “CVD method” hereinafter).
Particularly, the hard film used for the cutting tool is required to have wear resistance and heat resistance (oxidation resistance at high temperatures). In recent years, therefore, titanium aluminum nitride (TiAlN) having both the properties has been frequently used as a coating material for a cemented carbide tool in which the cutting edge reaches a high temperature during cutting. The reason why TiAlN exhibits the excellent characteristics is that the heat resistance is improved by the action of aluminum contained in a film, and stable wear resistance and heat resistance can be maintained up to a high temperature of about 800° C.
In some cases, the cutting edge of a cutting tool reaches a high temperature of 1000° C. or more during cutting. Under such a condition, sufficient heat resistance cannot be secured only by the TiAlN film, and thus an alumina layer is further formed on the TiAlN film to secure heat resistance, as disclosed in U.S. patent application Ser. No. 5,879,823.
Alumina has various crystal structures depending upon temperatures, but all the crystal structures are thermally metastable. However, when the temperature of the cutting edge of the cutting tool significantly changes over a wide range from room temperature to 1000° C. or more during cutting, the crystal structure of alumina changes to disadvantageously cause cracking or delamination in the film. However, in alumina having a α-crystal structure formed by a CVD method in which the substrate temperature is increased to 1000° C. or more, the thermally stable structure is maintained after the formation regardless of temperature. Therefore, in order to impart heat resistance to the cutting tool, it is effective to coat the cutting tool with an alumina film having a α-crystal structure.
However, as described above, in order to form alumina having a α-crystal structure the substrate must be heated to 1000° C. or more, and thus only limited substrates can be used. This is because a certain type of substrate is possibly softened to lose suitability for a substrate for wear-resistant members when being exposed to a high temperature of 1000° C. or more. Also, a high-temperature substrate such as a cemented carbide disadvantageously causes deformation when being exposed to such a high temperature. The hard film such as the TiAlN film formed as a film exhibiting wear resistance on the substrate generally has a maximum practical temperature of about 800° C., and thus the film possibly changes in properties to degrade the wear resistance when being exposed to a high temperature of 1000° C. or more.
As a technique for solving the above problem, Japanese Unexamined Patent Application Publication No. 2002-53946 discloses a method in which an underlying layer comprising a corundum structure (α-crystal structure) oxide film having a lattice constant of 4.779 Å to 5.000 Å and a thickness of at least 0.005 μm is formed, and an alumina film having a α-crystal structure is formed on the underlying layer. The component of the oxide film is preferably Cr2O3, (Fe, Cr)2O3, or (Al, Cr)2O3. When the component of the oxide film is (Fe, Cr)2O3, (Fex, Cr(1-x))2O3 (wherein x is 0≦x≦0.54) is preferably used. When the component of the oxide film is (Al, Cr)2O3, (Aly, Cr(1-y))2O3 (wherein y is 0≦y≦0.90) is preferably used.
It is also disclosed that crystalline α-alumina can be formed at a low substrate temperature by a method comprising forming a hard film comprising a compound nitride film composed of Al and at least one element selected from the group consisting of Ti, Cr, and V, forming an intermediate layer comprising a film of (Alz, Cr(1-z))N (wherein z is 0≦z≦0.90), oxidizing the intermediate layer to form a corundum structure (α-crystal structure) oxide film, and then forming a α-alumina film on the oxide film.
The above publication also discloses an example of this method in which a CrN film is formed on a substrate and then oxidized to form corundum structure Cr2O3, and then alumina mainly having a α-structure is formed on Cr2O3. However, as a result of research of these techniques, the inventors found that the techniques require heating the substrate to about 700° C. to 750° C. in an oxidizing atmosphere, and the CrN film easily delaminates from the substrate after this heating step. Therefore, further improvement is required for increasing adhesion between the substrate and the multilayered film.
The inventors already have proposed a film having excellent heat resistance and wear resistance, the film being formed by forming a TiAlN film as a hard film on the surface of a cutting tool, and further forming an alumina film mainly having a α-crystal structure on the TiAlN film (Japanese Patent Application No. 2002-231954). Specifically, the inventors have proposed that the hard film (TiAlN film) is formed on a substrate, the surface of the hard film is oxidized to form an oxide-containing layer on the surface of the hard film, and then an alumina film mainly having a α-crystal structure on the oxide-containing layer.
This method can improve productivity in comparison with the method disclosed in Japanese Unexamined Patent Application Publication No. 2002-53946 in which the TiAlN film is formed, the CrN film is formed and then oxidized to form corundum structure Cr2O3, and then the alumina film mainly having a α-crystal structure is formed. It is also possible to avoid cutting performance from being decreased due to Cr-containing films such as a Cr2O3 layer and a (CrN+Cr2O3) compound layer formed as the intermediate film.
However, when the alumina mainly having a α-structure is formed on the oxide-containing layer, formed by oxidizing the surface of the hard film (TiAlN film), the temperature of the substrate must be increased to about 700° C. to 750° C. in an oxidizing atmosphere. Like in the method disclosed in Japanese Unexamined Patent Application Publication No. 2002-53946, the hard film (TiAlN film) possibly delaminates from the substrate. Therefore, further improvement is required for increasing adhesion between the substrate and the multilayered film.